In recent years, with the increased speed of radio communication, transmission signals each having a wider bandwidth and a higher dynamic range has been improved. Under these circumstances, in order to reduce the degradation of the quality of signals to the minimum, electric power amplifiers having high linearity are needed. Furthermore, at the same time, from the viewpoint of reducing the size of devices and the operation cost and from the viewpoint of environmental issues and the like, there is also a growing need for electric power amplifiers operated with high electric power conversion efficiency.
In commonly used electric power amplifiers, the linearity and the electric power conversion efficiency have a conflicting relationship with each other. For example, it is possible to minimize the occurrence of out-of-band distortion by operating an electric power amplifier in a linear region in which saturation electric power is backed off. However, in this case, the electric power conversion efficiency is considerably reduced, thus resulting in an increase in electrical power consumed in the electric power amplifiers. Therefore, in order to satisfy both the linearity and the electric power conversion efficiency, the linearity is maintained by operating the electric power amplifier in a nonlinear region in which the electric power conversion efficiency is high and by using distortion compensation that removes nonlinear distortion that is generated at that time. A predistortion technique (hereinafter, referred to as a “PD technique”), which is one of modes of the distortion compensation, is a technology for enhancing the linearity of an output of an electric power amplifier by previously adding distortion having the inverse characteristic of nonlinear distortion generated in the electric power amplifier to a transmission signal.
As a method for previously adding distortion to a transmission signal in the PD technique, there are two methods, i.e., a method that uses a look-up table (LUT) and a method that uses a series. In a case of using a LUT, a distortion compensation coefficient associated with a transmission signal is read from the LUT and then the obtained distortion compensation coefficient is multiplied by the transmission signal. In contrast, in a case of using a series, a distortion compensation coefficient associated with a transmission signal is calculated by a series and then the obtained distortion compensation coefficient is multiplied by the transmission signal. The distortion compensation coefficients stored in the LUT and the coefficients included in the series are updated at any time by using, for example, the Least Mean Square (LMS) algorithm.
Patent Document 1: Japanese Laid-open Patent Publication No. 2009-213113
Patent Document 2: Japanese Laid-open Patent Publication No. 2014-132727
Incidentally, an electric power amplifier is sometimes modeled as a circuit that generates, as in the model represented by, for example, the Wiener model, linear distortion and nonlinear distortion. In this way, when applying the PD technique to the electric power amplifier that generates the linear distortion and the nonlinear distortion, it takes time to perform an update process on the LUT or the series. Namely, when a memory polynomial is formed by the LUT or the series, an update process related to a plurality of LUTs or a series is performed and, thus, the time needed for convergence of the distortion compensation coefficients stored in the LUTs or the coefficients included in the series is increased. Furthermore, if it takes time to perform the update process on the LUTs or the series, it is difficult to follow the variation in the characteristics of the electric power amplifier generated due to an environment factor, such as a temperature, to the distortion compensation and thus the performance of the distortion compensation is degraded.
Thus, to reduce the convergence time in the update process, it is conceivable to individually compensate the linear distortion and the nonlinear distortion of the electric power amplifier and individually obtain a distortion compensation coefficient of each of the pieces of distortion. Specifically, for example, it is conceivable to compensate the linear distortion by using an equalizer that is formed by a finite impulse response (FIR) filter, whereas it is conceivable to compensate the nonlinear distortion by using a LUT or a series. In this case, by performing an update process on tap coefficients of the FIR filter and by performing an update process on a single LUT or a series, it is possible to implement distortion compensation with high accuracy and it is thus possible to follow the distortion compensation to the variation in characteristics of the electric power amplifiers.
However, if the distortion compensation coefficients of the linear distortion and the nonlinear distortion are individually obtained, there is a problem in that both of the distortion compensation coefficients interfere with each other and it is thus difficult to uniquely obtain the distortion compensation coefficients. Furthermore, for example, due to an arithmetic calculation error or the like, the obtained distortion compensation coefficients may possibly be drifted.
For example, if a distortion compensation coefficient of nonlinear distortion is expressed by a series, a signal Xi obtained by performing nonlinear distortion compensation on a transmission signal xi at time i by using a series is expressed by Equation (1) below.Xi=(b1+b3|xi|2+)xi  (1)
where, in Equation (1), b1 and b3 denote coefficients included in the series. Furthermore, |x| denotes the amplitude of the transmission signal x. Then, a signal ui obtained by performing linear distortion compensation on a signal Xi by an equalizer is expressed by Equation (2) below.ui=a1Xi−1+a2Xi+a3Xi+1  (2)
where, in Equation (2), a1, a2, and a3 denote tap coefficients of the FIR filter. When Equation (1) is substituted in Equation (2) indicated above, Equation (3) below can be obtained.ui=(a1b1+a1b3|xi−1|2+a1b5|xi−1|4)xi−1+(a2b1+a2b3|xi|2+a2b5|xi|4)xi+(a3b1+a3b3|xi+1|2+a3b5|xi+1|4)xi+1  (3)
In this way, in each of the terms, the product of the tap coefficients a1, a2, and a3 and coefficients b1, b3, and b5 included in the series are expressed and the terms associated with the linear distortion and the terms associated with the nonlinear distortion interfere with each other. In other words, even if for example, the product of “a1b1” that optimizes the performance of distortion compensation is decided, it is difficult to specify each of the coefficients “a1” and “b1”. Therefore, it is difficult to uniquely decide the tap coefficients and the coefficients included in the series, thereby a stable distortion compensation operation may possibly be damaged. This problem also occurs in a case in which the distortion compensation coefficients of nonlinear distortion are read from a LUT.